With the development of CNNs, a new architecture called Fully Convolutional Neural Network (FCN) [1]} obtained large performance improvement for segmentation which also encouraged the emergence of U-Net [2]}. [3]} used deep dilated CNNs for segmentation of OARs, where the network has a larger receptive field. Other researchers [4]}, [5]} presented a fully convolution network-based model with auxiliary paths strategy which allows the model to learn discriminative features to obtain a higher segmentation performance. To deal with the imbalance problem, [6]} used one CNN for OAR detection and then another CNN for fine segmentation. Based on 3D-UNet [7]}, [8]} proposed FocusNet by adding additional sub-networks for better segmentation of small organs.
